Add multiple gitlab-monitor DB metrics to postgres-exporter

This ports metrics from the following database collectors in
gitlab-monitor:

- BlockedQueriesCollector
- SlowQueriesCollector
- StuckIdleInTransactionsCollector
- VacuumQueriesCollector

The ported queries sometimes have to look somewhat different to work in
the postgres-exporter, and hopefully they have the same or close-enough
outcomes as in gitlab-monitor.

Some of the metric names didn't adhere to best practices in gitlab-monitor
(like "_total" as a suffix for gauges), but for now I'm keeping them the
same in postgres-exporter to keep the number of changing things lower.

files/gitlab-cookbooks/gitlab/templates/default/postgres-queries.yaml

@@ -160,3 +160,80 @@ pg_stat_statements:
     - temp_blks_written:
         usage: "COUNTER"
         description: "Total number of temp blocks written by the statement"
+
+pg_blocked_queries:
+  query: |
+    SELECT
+      count(blocked.transactionid) AS total,
+      '__transaction__' AS table
+    FROM pg_catalog.pg_locks blocked
+    WHERE NOT blocked.granted AND locktype = 'transactionid'
+    GROUP BY locktype
+    UNION
+    SELECT
+      count(blocked.relation) AS total,
+      blocked.relation::regclass::text AS table
+    FROM pg_catalog.pg_locks blocked
+    WHERE NOT blocked.granted AND locktype != 'transactionid'
+    GROUP BY relation
+  metrics:
+    - total:
+        usage: "GAUGE"
+        description: "The current number of blocked queries"
+    - table:
+        usage: "LABEL"
+        description: "The table on which a query is blocked"
+
+pg_slow_queries:
+  query: |
+    SELECT COUNT(*) AS total
+    FROM pg_stat_activity
+    WHERE state = 'active' AND (now() - query_start) > '1 seconds'::interval
+  metrics:
+    - total:
+        usage: "GAUGE"
+        description: "Current number of slow queries"
+
+# TODO: In gitlab-monitor, missing age_in_seconds (nil) gets converted to 0.0,
+# here it results in an NaN. Should that be changed to the old behavior?
+pg_vacuum:
+  query: |
+    SELECT
+      COUNT(*) AS count,
+      MAX(EXTRACT(EPOCH FROM (clock_timestamp() - query_start))) AS age_in_seconds
+    FROM pg_catalog.pg_stat_activity
+    WHERE state = 'active' AND trim(query) ~* '\AVACUUM (?!ANALYZE)'
+  metrics:
+    - count:
+        usage: "GAUGE"
+        description: "The current number of VACUUM queries"
+    - age_in_seconds:
+        usage: "GAUGE"
+        description: "The current maximum VACUUM query age in seconds"
+
+# TODO: In gitlab-monitor, missing age_in_seconds (nil) gets converted to 0.0,
+# here it results in an NaN. Should that be changed to the old behavior?
+pg_vacuum_analyze:
+  query: |
+    SELECT
+      COUNT(*) AS count,
+      MAX(EXTRACT(EPOCH FROM (clock_timestamp() - query_start))) AS age_in_seconds
+    FROM pg_catalog.pg_stat_activity
+    WHERE state = 'active' AND trim(query) ~* '\AVACUUM ANALYZE'
+  metrics:
+    - count:
+        usage: "GAUGE"
+        description: "The current number of VACUUM ANALYZE queries"
+    - age_in_seconds:
+        usage: "GAUGE"
+        description: "The current maximum VACUUM ANALYZE query age in seconds"
+
+pg_stuck_idle_in_transactions:
+  query: |
+    SELECT COUNT(*) AS total
+    FROM pg_stat_activity
+    WHERE state = 'idle in transaction' AND (now() - query_start) > '10 minutes'::interval
+  metrics:
+    - total:
+        usage: "GAUGE"
+        description: "Current number of queries that are stuck being idle in transactions"